The majority of combustible liquid fuel used in the world today is derived from crude oil. However, there are several limitations to using crude oil as a fuel source. For example, crude oil is in limited supply, includes aromatic compounds believed to cause cancer, and contains sulfur and nitrogen-containing compounds that can adversely affect the environment.
Alternative sources for developing combustible liquid fuel are desirable. One option is to convert natural gas to liquid fuel or other chemical products. The conversion of natural gas to liquid fuel typically involves converting the natural gas, which is mostly methane, to synthesis gas, or syngas, which is a mixture of carbon monoxide and hydrogen. Fischer-Tropsch synthesis is an example of a hydrocarbon synthesis that converts the syngas to higher molecular weight hydrocarbon products. An advantage of using fuels prepared from syngas is that they typically do not contain nitrogen and sulfur and generally do not contain aromatic compounds. Accordingly, they have less health and environmental impact than conventional petroleum liquids based fuels.
Fischer-Tropsch synthesis products tend to include a large amount of high molecular weight linear paraffins (wax), which can be hydrocracked to form lower molecular weight products, and optionally subjected to additional hydroprocessing steps.
Multiple catalyst beds with intermediate cooling stages are commonly used to control the extremely exothermic hydrocracking reaction. Multiple beds are used to introduce a cooling quench (typically a gas, most often a hydrogen-containing gas) and to allow re-mixing of the reactor gas and liquids to allow for more efficient catalyst utilization and smoother/safer operation. Numerous means are known for mixing the gas/gas, liquid/liquid and gas/liquid mixtures between catalysts beds, examples of which are described in U.S. Pat. No. 5,837,208 and U.S. Pat. No. 5,690,896, the contents of which are hereby incorporated by reference.
In typical hydrocracking reactions, a feed is preheated and introduced along with a significant stream of a hydrogen-containing gas at the top of the reactor. A stream of relatively cool hydrogen-containing gas is introduced between the beds to provide the desired quenching (cooling) of the exothermic hydrocracking reactions. Gas has been a preferred quench fluid because of the relative ease in distributing across the reactor cross-section, its function as a reactant in the upgrading process, and the availability at reactor conditions downstream of the plant recycle and/or make-up hydrogen compressors. A limitation of using gases as the quench fluid is its relatively low heat capacity. Additionally, the use of hydrogen requires the presence of relatively expensive recirculation pumps.
It would be advantageous to provide methods for using additional quench streams when hydrocracking FT synthesis products. The present invention provides such methods.
The present invention is directed to a method for hydroprocessing Fischer-Tropsch products. The invention in particular relates to an integrated method for producing liquid fuels from a hydrocarbon stream provided by Fischer-Tropsch synthesis. The method involves separating the Fischer-Tropsch products into a light fraction with normal boiling points below 700xc2x0 F. and including predominantly C5-20 components and a heavy fraction with normal boiling points above 650xc2x0 F. and including predominantly C20+ of components. The heavy fraction is subjected to hydrocracking conditions, preferably through multiple catalyst beds, to reduce the chain length. The light fraction is used as all or part of a quench fluid between each catalyst bed.
The products from the hydroprocessing reaction can be separated into at least a hydrogen-rich gas stream, a distillate product predominantly in the C5-20 range, and a bottoms stream. The bottoms stream can optionally be resubjected to the hydrocracking conditions to provide an additional light fraction, or used, for example, to prepare a lube base oil stock. When used to prepare a lube base oil stock, it can be subjected to catalytic dewaxing and/or solvent dewaxing conditions.
In one embodiment, the heavy fraction and/or the light fraction include hydrocarbons in the same range derived from other sources, for example, petroleum refining.